Fatty Acid SynthesisDR MUHAMMAD MUSTANSAR

Fatty acids Fatty acids are a class of compounds containing a

long hydrophobic hydrocarbon chain and a terminal carboxylate group

They exist free in the body as well as fatty acyl esters in more complex molecules such as triglycerides or phospholipids.

Fatty acids can be oxidized in all tissues, particularly liver and muscle to provide energy

They are also structural components of membrane lipids such as phospholipids and glycolipids.

Esterified fatty acids, in the form of triglycerides are stored in adipose cells

Fatty acids are also precursors of Eicosanoids

Sources of Fatty acidsDietAdipolysisDe novo synthesis(from

precursors)- Carbohydrates, protein, and other molecules obtained from diet in excess of the body’s need can be converted to fatty acids, which are stored as triglycerides

Biosynthesis of Fatty acids

The excess dietary Carbohydrates & Proteins can be converted to fatty acids and are stored as Tri acyl Glycerol.Denovo synthesis of Fatty acids takes place in Liver, Kidney, adipose tissue and Lactating Mammary glands.Site: Cytoplasm of the cellRequirements: Acetyl CoA – source of Carbon atomsNADPH – provides reducing equivalentsATP – energy

═ Fatty acid synthesis in 3 stages

(i) Production of Acetyl CoA & NADPH

(ii) Conversion of acetyl CoA to Malonyl CoA

(iii) Reactions of Fattyacid synthase complex.

De novo fatty Acid Synthesis- IntroductionFatty acids are synthesized by an extra mitochondrial systemThis system is present in many tissues, including liver, kidney, brain, lung, mammary gland, and adipose tissue.Acetyl-CoA is the immediate substrate, and free palmitate is the end product.Its cofactor requirements include NADPH, ATP, Mn2+, biotin, and HCO3

– (as a source of CO2).

Location of fatty acid synthesis

FA synthase complex is found exclusively in the cytosol.

The location segregates synthetic processes from degradative reactions.

Sources of NADPHNADPH is involved as donor of reducing equivalents The oxidative reactions of the pentose phosphate pathway are the chief source of the hydrogen required for the reductive synthesis of fatty acids. Tissues specializing in active lipogenesis—ie, liver, adipose tissue, and the lactating mammary gland—possess an active pentose phosphate pathway. Other sources of NADPH include the reaction that converts malate to pyruvate catalyzed by the "Malic enzyme" (NADP malate dehydrogenase) and the extra mitochondrial isocitrate dehydrogenase reaction (probably not a substantial source, except in ruminants).

Acetyl co A- Sources and FateAcetyl co A, the precursor for fatty acid synthesis is produced from pyruvate, ketogenic amino acids, fatty acid oxidation and by alcohol metabolism

It is a substrate for TCA cycle and a precursor for fatty acids, ketone bodies and sterols.

Transportation of Acetyl co A Fatty acid synthesis requires considerable amounts of acetyl-CoANearly all acetyl-CoA used in fatty acid synthesis is formed in mitochondria Acetyl co A has to move out from the mitochondria to the cytosol

Cytosol – site of acetate utilization

Mitochondria – site of acetate synthesis

Transportation of Acetyl co AAcetate is shuttled out of mitochondria as citrateThe mitochondrial inner membrane is impermeable to acetyl-CoAIntra-mitochondrial acetyl-CoA first reacts with oxaloacetate to form citrate, in the TCA cycle catalyzed by citrate synthaseCitrate then passes into the cytosol through the mitochondrial inner membrane on the citrate transporter.In the cytosol, citrate is cleaved by citrate lyase regenerating acetyl-CoA.

Transportation of Acetyl co A

Enzymes and cofactors involved in the process of Fatty acid synthesis

Two main enzymes-Acetyl co A carboxylaseFatty acid SynthaseBoth the enzymes are multienzyme complexesCoenzymes and cofactors are-BiotinNADPHMn++

Mg++

Steps in Fatty acid synthase complex

1) Acetyl CoA is transferred to ACP by

Acetyl CoA ACP transacylase (CoA

is removed). The two carbon unit

acetate which is attached to ACP is

shifted to cysteine residue of keto

acyl synthase enzyme.

2) Now Malonyl CoA is transferred to

ACP by Malonyl CoA ACP

transacylase where Co A is removed

3) The acetyl unit (2) which is attached

to cysteine combines with malonyl

unit (3 ‘C’) where Co2 is released to

form keto acyl, ACP, Enzyme is keto

acyl synthase.

4) Ketoacyl ACP undergoes reduction

to form β-hydroxy acyl ACP, enzyme

is keto Acyl reductase NADPH

provides Hydrogens.

5) β-hydroxy acyl ACP undergoes

dehydration where one water

molecule is removed to form Enoyl

ACP (double bond between 2 & 3

Carbons) enzyme is dehydratase.

6) Enoyl ACP undergoes reduction to

form acyl ACP or butryl ACP,

enzyme is enoyl ACP reductase,

NADPH provides Hydrogens.

7) The 4 carbon butryl acid attached to

ACP is shifted to cysteine residue and

reactions 2 – 6 are repeated

For 1 cycle carbon chain length increase by 2 carbons

(2) Acetyl acid 1 cycle(4) Butyric acid 2 cycle(6) Caproic acid 3 cycle(8) Caprylic acid

Series of Reactions

After activation, the processes involved are-1. Condensation2. Reduction3. Dehydration4. ReductionThese steps are repeated till a fatty acid with 16 carbon atoms is synthesized

Repetition of these four steps leads to fatty acid synthesis

The overall reaction for the synthesis of palmitate from acetyl-CoA can be considered in two parts.

Part 1

First, the formation of seven malonyl-CoA molecules:

7Acetyl-CoA + 7CO2 + 7ATP

7malonyl CoA + 7ADP + 7Pi

Part 2

Then the seven cycles of condensation and reduction

Acetyl-CoA + 7malonyl-CoA + 14NADPH + 14H+ palmitate + 7CO2 + 8CoA +

14NADP+ + 6H2O

The biosynthesis of FAs requires acetyl-CoA and the input of energy in the form of ATP and reducing power of NADPH.

But malonate is made from acetyl CoA

7 acetyl-CoA + 7 CO2 + 7 ATP 7 malonyl-CoA

+ 7 ADP + 7 Pi + 7 H+

So overall:

8 acetyl-CoA + 7 ATP + 14 NADPH palmitate +14 NADP+ + 8 CoASH + 6 H2O + 7 ADP + 7 Pi

The NADPH comes from the pentose phosphateshunt. The source of acetyl-CoA is more complex.

Rxn for palmitate synthesis is:

Acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 7 H+

palmitate + 7 CO2 + 14 NADP+ + 8 CoASH + 6H2O

Comparison of β-Oxidation & Fatty Acid Synthesis

Βeta Oxidation pathway

Fatty acid Synthesis

Location Mitochondrial Cytoplasmic

Acyl Carriers(Thiols) Coenzyme A 4’ Phosphopantetheine and Cysteine

Electron acceptors and donors

FAD/NAD NADPH

OH Intermediates L D

2 Carbon product/donor

Acetyl co A Acetyl co A/ Malonyl co A